This invention relates to a motor control system and, more particularly, to such a system wherein a potentiometer is used to control the speed and direction at which a motor operates, and wherein an operating potential of only a single polarity need be used with the potentiometer in determining the direction at which the motor operates.
Bidirectional motors admit of various and divers applications. In a typical application thereof, the motor should be capable of operating at a desirable speed and either in a forward or reverse direction. Furthermore, if the motor is controlled by a manually adjustable device, such as a potentiometer, there are certain circumstances wherein the motor should exhibit zero speed when the adjustable element, or potentiometer, is at or very close to a zero speed setting. For example, when the motor is used to drive an article to a particular location, it is advantageous to be able to operate the motor at a relatively high speed until the article approaches the predetermined location, and then to drive the motor at a progressively decreasing speed so as to be able to stop the motor abruptly when the article reaches its predetermined location. In the event of an overshoot, it also is advantageous to drive the motor in a reverse direction so as to return the article to its predetermined location.
One type of motor control system which uses a potentiometer to determine the speed and direction of the motor is described in U.S. Pat. No. 3,950,683, issued to W. P. Lamson. In this system, the potentiometer is provided with both positive and negative operating potentials. The wiper of the potentiometer is adjustable so as to be set either at a zero speed setting, which may correspond to the mid-point of the potentiometer resistance, or at any location between the zero speed setting and either terminal of the potentiometer. Thus, when the wiper is set at a location between the zero speed setting and the potentiometer terminal to which the positive operating potential is supplied, the magnitude of the voltage at the wiper determines the speed at which the motor operates, and the positive polarity of this voltage is used to drive the motor in, for example, the forward direction. Similarly, if the wiper is set at a location between the zero speed setting and the potentiometer terminal at which the operating potential of negative polarity is supplied, then the magnitude of the voltage at the wiper determines the motor speed, and the negative polarity of this voltage is used to drive the motor in, for example, the reverse direction. The motor control system described in this patent also includes a dead-band circuit which prevents undesired slow motor speed, such as motor creeping, when the wiper is set to, or very close to, its zero speed setting.
A significant disadvantage of the motor control system described in the aforementioned patent is the requirement that, in order to control the motor direction, operating potentials of positive and negative polarities must be applied to the potentiometer terminals. By requiring such positive and negative polarity operating potentials, the power supply circuit which is needed to derive these operating potentials is relatively complex.